Galaxy intrinsic alignment (IA) is a source of both systematic contamination of cosmic shear measurement and its cosmological applications and valuable information on the large-scale structure of the universe and galaxy formation. The self-calibration (SC) method was designed to separate IA from cosmic shear, free of IA modeling. It was first successfully applied to the KiDS450 and KV450 data. We improve the SC method in several aspects and apply it to the DECaLS DR3 shear + photo-z catalog and significantly improve the IA detection to ∼14σ. We find a strong dependence of IA on galaxy color, with strong IA signal (∼17.6σ) for red galaxies, while the IA signal for blue galaxies is consistent with zero. The detected IAs for red galaxies are in reasonable agreement with the nonlinear tidal alignment model, and the inferred IA amplitude increases with redshift. Our measurements rule out the constant IA amplitude assumption at ∼3.9σ for the red sample. We address the systematics in the SC method carefully and perform several sanity checks. We discuss various caveats, such as redshift/shear calibrations and possible improvements in the measurement, theory, and parameter fitting that will be addressed in future works.
Intrinsic alignments (IA) of galaxies have been recognized as one of the most serious contaminants to weak lensing. These systematics need to be isolated and mitigated in order for ongoing and future lensing surveys to reach their full potential. The IA self-calibration (SC) method was shown in previous studies to be able to reduce the GI contamination by up to a factor of 10 for the 2-point and 3-point correlations. The SC method does not require the assumption of an IA model in its working and can extract the GI signal from the same photo-z survey offering the possibility to test and understand structure formation scenarios and their relationship to IA models. In this paper, we study the effects of the IA SC mitigation method on the precision and accuracy of cosmological parameter constraints from future cosmic shear surveys LSST, WFIRST and Euclid. We perform analytical and numerical calculations to estimate the loss of precision and the residual bias in the best fit cosmological parameters after the self-calibration is performed. We take into account uncertainties from photometric redshifts and the galaxy bias. We find that the confidence contours are slightly inflated from applying the SC method itself while a significant increase is due to the inclusion of the photo-z uncertainties. The bias of cosmological parameters is reduced from several-σ, when IA is not corrected for, to below 1-σ after SC is applied. These numbers are comparable to those resulting from applying the method of marginalizing over IA model parameters despite the fact that the two methods operate very differently. We conclude that implementing the SC for these future cosmic-shear surveys will not only allow one to efficiently mitigate the GI contaminant but also help to understand their modeling and link to structure formation.
Concentration is one of the key dark matter halo properties that could drive the scatter in the stellar-to-halo mass relation of massive clusters. We derive robust photometric stellar masses for a sample of brightest central galaxies (BCGs) in SDSS redMaPPer clusters at 0.17 < z < 0.3, and split the clusters into two equal-halo mass subsamples by their BCG stellar mass $M_*^{\mathrm{BCG}}$. The weak lensing profiles ΔΣ of the two cluster subsamples exhibit different slopes on scales below 1 h−1 Mpc. To interpret such discrepancy, we perform a comprehensive Bayesian modelling of the two ΔΣ profiles by including different levels of miscentring effects between the two subsamples as informed by X-ray observations. We find that the two subsamples have the same average halo mass of 1.74 × 1014 h−1 M⊙, but the concentration of the low-$M_*^{\mathrm{BCG}}$ clusters is $5.87_{-0.60}^{+0.77}$, ∼1.5σ smaller than that of their high-$M_*^{\mathrm{BCG}}$ counterparts ($6.95_{-0.66}^{+0.78}$). Furthermore, both cluster weak lensing and cluster-galaxy cross-correlations indicate that the large-scale bias of the low-$M_*^{\mathrm{BCG}}$, low-concentration clusters are ${\sim }10{{\ \rm per\ cent}}$ higher than that of the high-$M_*^{\mathrm{BCG}}$, high-concentration systems, hence possible evidence of the cluster assembly bias effect. Our results reveal a remarkable physical connection between the stellar mass within 20 − 30 h−1 kpc, the dark matter mass within ∼200 h−1 kpc, and the cosmic overdensity on scales above 10 h−1 Mpc, enabling a key observational test of theories of co-evolution between massive clusters and their central galaxies.
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